1. Field of the Invention
This invention relates to polyphenylene ether based thermoplastic compositions.
2. Discussion of the Background
Polyphenylene ethers ("PPEs"), also called polyphenylene oxides, are polymers with good thermal and electrical properties. In particular, poly(2,6-dimethyl-1,4-phenylene) ether has become industrially important as a component of thermoplastic molding compositions. However, there are also problems with PPEs.
For example, pure PPEs are difficult to process due to their high melt viscosity Molded pieces produced from PPEs have high hot shape stability, but their notch sensitivity is too high for most applications.
There have been numerous attempts to improve the processibility and impact strength of PPE molded pieces (see German Patent No. 1,694,255 (corresponding to U.S. Pat. No. 3,361,851), German Patent No. 1,694,257 (corresponding to U.S. Pat. No. 3,383,435), and German Patent No. 1,694,290 (corresponding to U.S. Pat. No. 3,379,792)). And, it has been found that the mere addition of polyolefins, polystyrenes, and/or polyamides does not provide the optimization of the properties of PPE-containing molding compositions as would be desired.
Mixtures of PPEs with high impact polystyrenes have become quite important industrially (see German Patent No. 2,211,005). For example, see the thermoplastic compositions disclosed in German Patent No. 2,119,301 which are comprised of PPEs and a rubber-modified polystyrene, or PPEs and a polystyrene and a rubber, or PPEs and a rubber-modified polystyrene and a polystyrene and a rubber. Improved impact strength is claimed for compositions in which the mean particle diameter of the dispersed elastomeric phase is under a maximum of 2 micron.
Polymer mixtures based on PPEs and having specific values of impact strength and softening temperatures are of substantial significance economically. In principle, such tailored mixtures can be prepared from PPEs and high impact polystyrenes with the polystyrenes containing various amounts of rubbers. Such an approach is costly, however, because a wide variety of types of polystyrene must be kept available in order to adjust the composition to a given set of desired qualities.
It is within the realm of possibility to incorporate rubber into polystyrene following preparation of the basic polystyrene polymer, but there is the drawback that the rubber is ordinarily available only in the form of bales, and thereby difficult to handle.
This drawback can be overcome by employing a so-called "interpolymer" instead of a rubber. Thus, for example, German AS No. 22 58 896 proposes to use an interpolymer which is a graft copolymer comprised of a diene rubber in an amount of 30-70 wt.% and a vinylaromatic compound or an alkyl acrylate ester in an amount of 70-30 wt.%. If this method is employed, various properties of the thermoplastic compositions can be controlled, but only by prior tailored synthesis of a special interpolymer.
It would thus be simpler if various types of thermoplastic compositions could be prepared by simply mixing together only a few starting components.
It is known that the impact strength of thermoplastic compositions based on PPEs can be improved by adding E-SBR rubbers (see Comparison Examples C and G, in the Table infra). Thus, e.g., thermoplastic compositions based on PPEs, styrene polymers, and powdered, filler-containing rubber (see German Patent Application No. P 34 24 219.8) are known.
On the other hand, additives comprising styrene-butadiene polymers prepared by emulsion polymerization, in which the polymers have a styrene content above 80 wt.%, have virtually no effect on the impact strength of PPE-containing thermoplastic compositions (see Comparison Examples A, B, and F, infra).
Hence there remains a strongly felt need for a thermoplastic composition which can be prepared simply and which is imbued with high dimensional stability under heat and high impact strength.